Volume meters on liquid dispensing pumps, such as gasoline and diesel pumps at filling (gas) stations or gas bars, require periodic calibration measurements to ensure that the volume displayed on a pump's meter accurately reflects the volume of liquid that is being dispensed from the pump. Significant overcharging (or undercharging) can result when the error in a pump's volume meter reading is beyond an allowable tolerance.
A bottom drain prover can be used to perform the calibration measurement for the pumps at a filling station. As shown in FIG. 1 bottom drain prover 100 typically has a narrow upper neck 100a with fill opening 100a′ for filling the prover with liquid from the liquid dispensing pump, a wider diameter body section 100b for accumulating and holding the bulk of the liquid pumped into the prover, and a narrow drain section 100c with a drain opening. Drain valve 100d is provided for controlling release of the accumulated liquid from the prover. The prover's volume meter can be a gauge glass tube 100e that has its interior volume connected to the interior volume of neck 100a. The gauge glass is marked along its height with a neck scale that reflects the capacity of liquid in the prover when drain valve 100d is closed and the prover is filled with liquid up to the neck region of the prover. The nominal capacity of the prover, and deviations there from, are marked on the scale using standards that are established by the applicable standards organization, such as, in the United States, the National Institute of Standards and Technology, or in Canada, Measurement Canada's Calibration Services Laboratory. If the nominal capacity of the prover is, for example, five gallons (or nominal conversion to twenty litres), five gallons (twenty litres), as measured by the pump's meter, of liquid from the dispensing pump is pumped into the prover, and the prover's calibrated neck meter is read to determine the deviation, if any, from five gallons (twenty litres). If the deviation exceeds an allowable tolerance, the pump's meter can be adjusted so that the pump's meter indicates a volume of liquid within the allowable tolerance as measured by the prover's neck scale. In an alternative meter calibration method, liquid may be pumped from the dispensing pump into the prover until the neck scale indicates that the prover is filled to nominal capacity. The pump's meter can then be recalibrated, if necessary, to read the pumped nominal capacity of liquid in the prover. One suitable bottom drain prover is a SERAPHIN brand special J prover, which is available from Pemberton Fabricators, Inc., Rancocas, N.J., USA.
Typically when provers are used to calibrate gas and diesel pumps at a filling station, one or more provers have their bottom drain(s) connected to a common or dedicated drain holding tank(s) so that after each filling of a prover, the prover can be drained for another filling. The combination of prover(s) and drain tanks(s) can be installed on a vehicle, or mounted on a trailer hitched to a vehicle for convenient transport.
As shown in FIG. 2(a) and FIG. 3 a mobile proving system may comprise holding tank 200 that can be structurally connected to a wheeled trailer or mounted on a truck bed (not shown in the figures) for selective attachment to a vehicle for transport to sites, such as gas stations where gasoline or diesel fuel pumps, or more generally, meter devices, require calibration. Prover 100 is similar to the prover shown in FIG. 1. Drain section 100c is connected to holding tank 200 so that opening drain valve 100d allows liquid in the prover's internal volume to drain into the holding tank. Tank discharge line 200a can be connected to a suitable device, such as butterfly valve 200b for controlling release of accumulated liquid from the holding tank. Vent conduit 400 connects the interior volume of neck 100a and tank 200 to the ambient environment surrounding the tank (atmosphere) via vent opening 400a. A breather cap assembly 300 can be provided for preventing foreign objects or liquids, such as rain, from entering fill opening 100a′ when the prover is not being used. As shown in FIG. 2(b) the breather cap assembly may comprise bayonet flange 300a suitably attached to the opening of the prover neck, for example, by neck flange 100f at the fill opening of the prover and nozzle sealing gasket 90, and breather cap 300b, which can be removably connected to the bayonet flange by a twisting motion. A suitable breather cap assembly comprises ELF 3 series filler breather (breather cap part no. 02080124 and associated bayonet flange part) available from HYDAC Technology Corporation, Bethlehem, Pa., USA. As seen in FIG. 2(c) through 2(e), through opening 90a in nozzle sealing gasket 90 is smaller than the through openings 300a′ and 100f in bayonet flange 300a and prover's neck flange 100f. Further opening 90a is sized to form a tight fit around the exterior of a pump's nozzle that is inserted through the opening. Typical nozzle sealing gasket 90 comprises a flexible material, such as a rubber composition, at least around through opening 90a so that the gasket seals around the nozzle inserted in through opening 90a. 
In operation of the bottom drain prover system shown in FIG. 2(a) and FIG. 3, with breather cap 300b removed as shown in FIG. 3, and prover drain valve 100d closed, a liquid dispensing pump's nozzle is inserted into fill opening 100a′ in neck 100a, that is, inserted through through opening 300a′ in bayonet flange 300; through opening 90a in nozzle sealing gasket 90; and through opening 100f in neck flange 100f, to fill the prover with liquid via the pump. Vapor from the liquid filling the prover can evaporate into the neck volume of the prover and connected vent conduit 400 during the filling process. If the liquid is highly volatile, the volume of liquid lost in the vapor can introduce a significant error in the measured volume in the prover that is not attributable to the dispensing pump's meter calibration. In other words the dispensing pump's meter may be within allowable tolerance while the prover system is indicating lower volume of pumped liquid to the prover due to evaporation of the liquid while filing the prover. Further after the pump's nozzle is withdrawn from the fill opening and the volume of liquid accumulated in the prover is released into the holding tank by opening drain valve 100d, some of the vapor/air mixture suctioned up through vent conduit 400 escapes to atmosphere through fill opening 100a′ while ambient air is drawn into the holding tank through vent opening 400a. 
Further the operator of the prover who inserts the nozzle from the dispensing pump into the fill opening of the prover will be in the vicinity of potentially hazardous gas or diesel vapors while the prover is being filled with liquid.
Therefore there is need for a liquid proving system that will prevent the release of vapors from the liquid being pumped into the prover so that the volume of liquid being pumped into the prover is not reduced by vapor losses prior to measurement of the volume with the prover's gauge, and for a liquid proving system that will not release vapors from the liquid being pumped into the prover in the vicinity of the operator of the prover.